ダイエット中です。
私はダイエット中です。
I've decided to go on a diet.
I decided to go on a diet.
You should go on a diet.
One month has passed since I started my diet.
I have to lose weight, so I'm on a diet.
I have to lose weight, so I'm on a diet.
of or relating to the diet
You must stick to your diet.